An understanding of cell-surface modifications that attend such processes as cellular differentiation and malignant transformation is dependent upon the biochemical characterization of membrane components in the undifferentiated and differentiated state. The research proposed here is designed to define the molecular basis for two such modifications in a simple and well-characterized membrane. During gametic differentiation in Chlamydomonas, the flagellar membrane acquires agglutinability via the lectin concanavalin A; it also acquires the ability to recognize and agglutinate with membranes from cells of the opposite mating type. The membrane component(s) involved in lectin agglutinability will be studied by their H3-con A binding capacity, by affinity chromatography, and by isolating mutant strains defective in acquiring agglutinability. The membrane component(s) involved in sex-specific recognition events will be analyzed by affinity chromatography and by a novel approach whereby glutaraldehyde-fixed cells are trypinized to remove peptides essential for the recognition process. Existing mutant strains defective in sexual agglutination are expected to facilitate this analysis. The project should elucidate how a eukaryotic membrane containing a very few species of glycoprotein can come to acquire new surface properties involved in intercellular interactions.